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3 la puterea 1+ 3 la puterea 2+3 la puterea 3+...+ 3 la puterea 2011

Răspuns :

S=3^1+3^2+3^3+....+3^2011 |·3
3S=3^2+3^3+3^4+...+3^2012
Scadem si obtinem:
2S=3^2012-3^1
S=(3^2012-3^1):2